The present invention relates to cryogenic separation of gases and, in particular, to a process and apparatus for the temporary supply of a back-up quantity of a “first” gas to maintain the level of production of the first gas from a cryogenic separation of a gaseous mixture comprising the first gas and at least one other gas in the event of reduction in the level of production of said first gas from the separation. The invention has particular application to the production of gaseous oxygen (“GOX”) from the cryogenic separation of air.
GOX may be produced in a cryogenic air separation unit (“ASU”). Such an ASU may be integrated with a downstream process that utilises the GOX in some way. For example, the GOX may be used in the production of synthesis gas (“syngas”) which is a mixture of hydrogen and carbon monoxide and which may be used in the preparation of higher molecular weight hydrocarbon compounds and/or oxygenates. A suitable example of a process to produce hydrocarbons would be the Fischer-Tropsch process. More than one ASU may be linked in parallel to produce GOX for the downstream process.
Some downstream processes, e.g. syngas production, gasification processes and ethylene oxide production, require a substantially constant level of production of GOX, that is the pressure or flow of the GOX must be maintained to within a narrow range. These processes are often referred to as “oxygen-critical processes”. Thus, back-up systems must be in place to ensure the constant supply of GOX in the event of a reduction in the pressure or flow of the GOX product for whatever reason. In this connection, the pressure or flow of the GOX product may decrease because a component of the ASU fails suddenly. For example, the main air compressor, a booster air compressor (if present), an air pre-purifier, a liquid oxygen (“LOX”) pump or a valve may fail.
It is well known to provide back-up GOX from a storage reservoir of high pressure (“HP”) LOX. In the event the pressure or flow of the GOX product drops below a certain level, LOX may be taken from the reservoir and vaporised in a vaporiser to produce back-up GOX at the required customer pressure. It is also well known to provide back-up GOX from a storage reservoir of low pressure (“LP”) LOX. In the event the pressure or flow of the GOX product drops below a certain level, LOX may be taken from the LP reservoir pumped to the desired pressure by one or more back-up LOX pumps and vaporised in a vaporiser to produce back-up GOX.
The back-up system is brought on line on receipt of a trigger signal, such as low product supply pressure. In the case of such a HP liquid back-up system, the trigger signal causes a vaporiser oxygen control valve to open. For the LP liquid back-up system, the trigger signal would also bring the, or each, back-up LOX pump to its design operating point. However, vaporisers cannot instantly attain their design vaporisation capacities when called upon to operate. The time taken to achieve that capacity depends on the type of vaporiser installed. Generally, ambient vaporisers have better response times than steam sparged water bath vaporisers due to relative inventories and unit masses. For example, a steam sparged water bath vaporiser must be kept warm so that it is ready for instantaneous use. Unfortunately, it is simply not possible initially to force LOX through the warm vaporiser at the design rate as the oxygen pressure drop through the vaporiser would be too high at warm standby conditions. The vaporiser needs time to cool down to a point where LOX may be vaporised at the necessary rate. This period of time may be up to 30 seconds within which time the oxygen-critical process may have been affected by the reduction in pressure or flow of GOX thereto.
It is well known to have a GOX buffer vessel in communication with the GOX output from the ASU(s) so that the GOX inventory of the line may be maintained high enough so that no unacceptable drop in line pressure occurs during the time taken for the vaporiser in the back-up system to come fully on-line. Such a buffer vessel may be at line pressure or may be pressurised, in which case a valve would have to used to reduce the pressure of the pressurised GOX before it would be released into the GOX product line. One drawback of using the buffer vessel is the capital cost involved.
WO-A-99/40304 (published on 12 Aug. 1999) comprises a combined cryogenic air separation unit/integrated gasifier combined cycle power generation system and describes a method for operating the ASU to vary its power consumption to maximise net power production during peak demand periods while maintaining peak efficiency when the power generation system operates at varying power production. The oxygen production rate is maintained at a stable optimum level throughout the day and is not subject to significant fluctuations during changes in power plant operating conditions. Referring to FIG. 1 of WO-A-99/40304, in periods of off-peak power demand, excess liquid oxygen generated by the ASU may be stored in the bottom of the low pressure distillation column 6 or transferred through line 13 to vessel 21 where it is stored until such time as it is needed during periods of high power demand in the integrated gasifier combined cycle system.
U.S. Pat. No. 6,062,044 (published on 16 May 2000) discloses the use of a liquid oxygen storage tank to store excess liquid oxygen which can be used to satisfy increases in oxygen demand.
It is an objective of the present invention to provide an alternative system for providing a back-up quantity of a first gas without having to use one or more expensive buffer vessels or at least to allow the capacity of such buffer volume to be substantially reduced. There is always an “inventory” (or store) of liquefied first gas in the cryogenic separation system, usually in the sump of a distillation column. The size of the inventory will depend on the size of the cryogenic distillation system but there is usually more than enough liquefied first gas stored in the distillation system itself to satisfy demand for the first gas during the time taken for the vaporiser in the main back-up system to fully come on-line. The inventors have devised a way of using this source of liquefied first gas to produce a back-up quantity of first gas and maintain the level of production of the first gas.